The term “cold seal” refers to sealing a package at ambient temperature, typically 15-26° C., as opposed to a high temperature sealant polymer that seals upon the onset of heat and pressure. Because of their ability to form seals without heat, cold seal cohesives are ideally suited to the packaging of heat sensitive products such as bakery and confectionery products. In addition, cold seal cohesives allow faster packaging speeds to be achieved relative to the heat seal method.
Cold seal cohesives are a class of adhesives that bond primarily to themselves by the application of pressure. These cohesives produce excellent face-to-face seals and adhere weakly or not at all with face-to-back contact. In conventional cold-seal cohesives, there are two essential components: a cohering component (natural or synthetic rubber) and a reinforcing polymer system component that stabilizes, promotes substrate adhesion, and influences the aggressiveness of the cold seal. Natural rubber in latex form is an extremely high molecular weight, branched and coiled polymer comprised largely of cis-polyisoprene. In a relaxed state, there are numerous molecular entanglements within the polymer chain and with adjoining polymer chains. When two surfaces of natural rubber are brought together, a minimum of energy is required to induce an interdiffusion with entanglements of surface molecules. This affinity that natural rubber molecules have for themselves characterizes the cohesive bond. Modifying components are added to natural rubber to improve adhesion to the substrate.
The performance of the cold seal can vary based on many factors. Cold seals are perishable and should be used within the suggested shelf life or the performance of the cold seal could be adversely affected. In addition, cold seal cohesives can also stratify in storage and should be stirred prior to use or the cold seal performance could be adversely affected. Also, cold seal cohesives typically have a pH in the 10.0-10.3 range. However, materials that have had a lot of mixing or excessive press time may exhibit a drop in pH. Typically, if the pH drops below 8.5 the cold seals may no apply and perform correctly. High heat and humidity storage of the coated products could adversely affect the performance of the cold seal. Finally, proper drying of the product will also affect its performance.
With most applications, the application of the cold seal is applied in a pattern around the perimeter of the lamination surface. These film laminations typically consists of 1) An outer print web film, 2) a film for direct application of the cold seal, and 3) an adhesive layer to bond the two films together. Typically, the film surface in which the cold seal cohesive is directly applied should preferably have very good adhesion to the cold seal cohesive. If this adhesion of the cohesive to the film substrate does not exist, the consequence will be package failure at the location where the cohesive is in contact with the film substrate. This failure will result in loss of hermetic seal and ultimately spoilage of the food product contained within the package. Cold seal coating weights of 2 to 4.5 lbs/ream satisfies most sealing requirements. One ream is 3,000 square feet.
Thus, it is preferred to provide a film that has excellent adhesion to the cold seal cohesive. It is also preferred to provide a film that is compliant with US Food and Drug Administration regulations.
U.S. Pat. No. 4,252,851 discloses an oriented, multiple-layer film comprising a core layer and a skin layer comprising a propylene/butene copolymer having 10-15% butene content. The function of this layer is as a heat seal layer and there is no subsequent surface treatment performed on this heat seal layer. So, this layer would not provide a good cold adhesion to a cold seal cohesive.
U.S. Pat. No. 5,482,780 discloses a multilayer biaxially oriented polypropylene film having an isotactic polypropylene polymer core, a cold seal release layer on one side of the core and a cold seal receptive layer on the opposite side of the core. The cold seal receptive layer consists of an ethylene-propylene random copolymer containing 2-8 wt. % ethylene. This layer can either be flame or corona treated to improve cold seal receptivity.
U.S. Pat. No. 5,900,294 discloses a biaxially oriented multilayer polyolefin film which can be heat-sealed at low temperatures. The film comprises a base layer and at least one outer layer comprising a propylene/butene copolymer of 20-30 wt. % butene. This film is intended for heat-seal at low temperatures and would not provide a good cold adhesion to a cold seal cohesive.
U.S. Pat. No. 6,022,612 discloses a multilayer biaxially oriented polypropylene film having an isotactic polypropylene polymer core and a cold seal receptive layer on the opposite side of the core. The cold seal receptive layer consists of 30% to 60% by weight of a thermoplastic rubber and between 40% and 70% by weight of a polyolefin polymer. The thermoplastic rubber is either a styrene-isoprene-styrene or styrene-butadiene-styrene copolymer. The polyolefin can be virtually any polyolefin from polypropylene, polyethylene, or copolymers. This patent does not specify another treated layer for metallization on the opposite side of the core. In addition, this patent also uses a different cold seal adhesion layer formulation than the present invention.
U.S. Pat. No. 6,165,599 discloses an oriented composite film comprised of a polypropylene core, having on one or both surfaces of the core skin layers comprised of a metallocene catalyzed polypropylene and a low molecular weight hydrocarbon resin. The purpose of this skin layer is to improve such properties as the film modulus, haze, and moisture barrier transmission. This patent does not teach the use of the metallocene polypropylene polymer for subsequent surface treatment and cold seal cohesive application.
International Application WO 00/40406 discloses a film structure comprising a first outer layer comprising a migratory additive, a second outer layer comprising a metallocene polyethylene and core layer in between the two outer layers. The metallocene polyethylene outer layer is laminated to a second film. This film does not disclose the subsequent treating or blending of the metallocene polyethylene skin layer.
In short, the prior art multi-layer polyolefin generally either contain a natural or thermoplastic rubber in the layer to which a cold seal cohesive is applied to do not provide good adhesion to cold seal cohesives or they do not provide good adhesion to cold seal adhesives/cohesives. Therefore, there is a need for a metallizable multi-layer polyolefin film that provides excellent adhesion characteristics to cold seal cohesive formulations.